I. Field of the Invention
The present invention relates generally to communications systems in which data is transmitted by user communications devices. More particularly, the present invention is directed to a novel system and method for reducing interference generated by a code division multiple access (CDMA) communications device.
II. Description of the Related Art
The use of variable rate encoding techniques in a code division multiple access (CDMA) system provides the ability to reduce the amount of information used to represent speech or other data as compared with fixed rate encoding techniques. For example, by using variable rate vocoding techniques fewer bits of information may be transmitted when speech levels are low. In a CDMA communication system as described in the Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard IS-95A entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", variable rate encoded speech or data is transmitted by the communication unit using frames of a fixed time duration. For data rates which are less than the maximum rate, the extra space available in the frame may carry signaling bits, if available, or secondary traffic bits if the communication unit is utilizing, for example, simultaneous voice and data service options. However, if there are not enough bits between primary traffic and signaling or secondary traffic to fill up an entire frame, the remaining blank space in the frame may be distributed among the transmitted bits in a pseudo-random manner, resulting in "bursts" of data being transmitted intermittently throughout the frame. During this blank space, the transmitter is gated off.
Alternatively, for low rate encoded speech or other data, the information bits may be repeated as necessary to fill the frame instead of turning off the transmitter. In such a case, the power of the transmitter is nonetheless typically reduced to conserve battery life and system capacity, since the data redundancy in the frame allows a satisfactory probability of successful reception at a lower signal to noise ratio.
As can be seen, the use of variable rate transmission techniques leads to significant variation in the transmit power level of the CDMA transmitter during each frame. The variation in power level may be a mere reduction in the case of repeated bits, or turning off in the case of pseudo-random burst distribution throughout the frame. This variation of the subscriber station's transmit power produces a transmitted signal which is capable of interfering with the operation of nearby electronic equipment. Because this variation modulates the amplitude of the RF energy transmitted, interference which is caused by such transmit power variation is referred to herein as "amplitude modulation interference", or simply "AM interference." It has been found that many electronic devices, such as hearing aids, tend to act as AM detectors, leading to interference being incurred during operation in the presence of transmitters which vary the amplitude of the transmitted signal.
Accordingly, what is needed is a transmission scheme for a CDMA communication system which minimizes the potential for interference with electronic devices.